Process of preparing para-hydroxybenzoic acid



Patented Nov. 28, 1933 PROCESS OF PREPARZENG PARA-HYDROXY- BENZOIC ACIDLindley E. Mills and William W. Allen, Midland,

Mich, assignors to The Dow Chemical Company, Midland, Mich,

igan

a corporation of Mich- No Drawing. Application July 5, 1932 Serial No.620,978

'7 Claims.

The invention relates to improvements in processes of preparingpara-hydroxybenzoic acid; and-has particular reference to processeswherein the foregoing compound is prepared from ortho-hydroxybenzoicacid (salicyclic acid) as a basic raw material.

The production of di-potassium ortho-hydroxybenzoate in a hydrated formby reacting potassium hydroxide with ortho-hydroxybenzoic l0 acid isknown.- It has also been disclosed that the hydrated orthocompoundrearranges upon heatingto about 240 C. to yield di-potassiumpara-hydroxybenzoate. This dipotasslum parahydrox-ybenzoate can be thenreadily acidified 16, to para-hydroxybenzoic acid. However, the yield ofpara-acid obtained by carrying out this process is approximately only 50per cent, based on the amount of ortho-acid used, because dipotassiumortho-hydroxybenzoate cannot be completely dehydratedat atmosphericpressure at any temperature below the temperature at which rearrangementto the para compound begins, and in the presence of even'very snrallquantities of molecularly retained water the rearrangement .25 isaccompanied bythe production of phenol and potassium carbonate asundesirable by-products.

It is therefore, among the objects of this invention to' provideimprovements in the foregoing process for the preparation ofpara-hydroxybenzoic acid whereby, in carrying out the process, theproduction ofphen'ol and potassium carbonateis substantially avoided andthe yield of the para acid is increased to practically quantitative. 7

We have now found that the foregoing objects can be accomplished bycompletely dehydrating the hydrated" di-potassium ortho-hydroxybenzoateby heating at a temperature below 240 C. under sub-atmospheric pressurefor a suitable 40 period of time, and then heating the anhydrous productin a substantially oxygen-free atmosphere at'a temperature of betweenabout 240 and 280 0., followed by acidification of the soprepareddi-potassium para-hydroxybenzoate.

To the accomplishment of the foregoing and related ends, the invention,then, consists of the process hereinafter fully described andparticularly pointed out in the claims, the following descriptionsetting forth in detail one mode of carrying out the invention, suchdisclosed mode illustrating, however, but one of various ways in whichthe principle of the invention may be used.

In carrying out our improved process for the 55.1 production ofpara-hydroxybenzoic acid, we prepare di-potassium ortho-hydroxybenzoatein the usual manner by reacting ortho-hydroxybenzoic acid with 2 mols ofpotassium hydroxide; then heat the di-potassium ortho compound at atemperature between about 130 and 240 C., preferably between about 180and 230 C. under sub-atmospheric pressure, preferably below about 100millimeters of mercury absolute, for a. sufficient length of time tocompletely dehydrate the said ortho-compound; then heat the anhydrousproduct in an atmosphere which contains no free oxygen, e. g. carbondioxide, nitrogen, etc, at a temperature of between about 240 and 280C.; and then subsequently acidity the di-potassium para-hydroxybenzoatethus obtained.

The following examples are illustrative of the result which can beobtained by carrying out our invention:

Example 1 A solution of a di-potassium ortho-hydroxybenzoate wasprepared by dissolving 14.2 pounds of 88 per cent potassium hydroxideand 15.6 pounds of ortho-hydroxybenzoic acid in 7.5 pounds of water.This solution was then dried at a temperature of about 190" C. under anabsolute pressure of between about 25 and 50 millimeters of mercury, fora period of 3 hours. At the end of this time carbon dioxide was led intothe drying vessel under a pressure slightly above atmospheric and thetemperature of the anhydrous ortho-hydroxybenzoate was raised to about240 0., where it was maintained for 3 hours. The product was thencooled, dissolved in 10 pounds of water, and the solution acidified withhydro chloric acid. The acid solution was filtered to obtain thepara-hydroxybenzoic acid, which was then dried in the air at about 90 C.The yield of the para-acid was 15 pounds, or above 96 per cent oftheoretical based on the 15.6 pounds of orthoacid used. The meltingpoint of the prodnot, without further purification, was 208-2l0 C.

Example 2 The same quantities of reactants as in Example 1 were againintermixed and subjected to a temperature of about C. under an absolutepressure of between about 25 and 50 millimeters of mercury for a periodof three hours. At the end of this period the pressure in the reactorwas maintained substantially the same but the temperature was raised toabout 260 C. and maintained thereat for three hours. The product wasthen worked up to obtain 14.9 pounds of para- 111? hydroxybenzoic acid,or a yield of 95.5 per cent of theoretical.

It is at once apparent from the foregoing examples that our improvementsin. processes of preparing para-hydroxybenzoic acid have the effect ofpractically doubling the yield obtained by former methods, and makepossible the direct production of a very high grade product.

It is particularly to be noted that it is not necessary that thehydrated ni-petsesium erthohydroxybenzoate be stirred or agitated duringdehydration thereof when such dehydration is accomplished in themannerwe'have set'i'orth. We have also found it advantageous, inconducting the process in cyclic fashion, to acidify the dipotassiumpara-hydroxybenzoate with sufficient ortho-hydroxybenzoic acid toreplace the potassium substituted in the hydroxyl group prior toacidifying with hydrochloric acid the remaining potassium substituted inthe carboxyl group. The potassium chloride and potassiumorthmhydroxybenz ate can then be separated readily. By carrying out thisprocedure it is apparent that onehalf of the potassium is recovered forsubsequent use in such form thatit does not require further treatment.This procedure cuts in half the amount of potassium chloride which-mustbe electrolyzed to recover the urn therefrom, and so results in adistinct saving. i

other modes of applying the principle of our invention may be employedinstead pd the'one explained, change being made as regards the processherein disclosed, provided the step or steps stated by any of thefollowing clladtlns or the equivalent of such stated step or steps beemployed.

We therefore particularly point out and distiznctly ciaim as ourinvention:-

1. In a process for the preparation of parahydroxybenzoic acid whereinhydrated di-potassium ortho-hydroxybenzoate is an intermediate product,the step which consists in completely dehydrating the said di-potassiumortho-hydroxybenzoate by heatingunder sub-atmospheric Pressure.

2.1m a process for the pnreparation' f parahydrexybenzoic acid whereinhydrated di potassium ortho-hydroxybenzoate is an intermediate product,the step which consmts completely dehydrating the said di-potassiumorthohydroxybenzoate by heating to a temperature of between about 130and 246 .C. under an absolute pressure below 100 millimeters of 3. In aprocess for the preparation of parahydroxyhenzoic acid wherein hydrated{ii-potassium ortho-hydroxybenzoate is an mtermediate productythe stepwhich-consists incompletely dehydrating the said di-apotasfiumortho-hydroxybenzoate by heatingto a temperature'of between about 180and 236 C. under an absolute pressure of between about 25 andlie-millimeters of mercury.

4. The process for the preparation of parahydroxy-benzoic acid whichcomprises reacting potassium hydroxide with ortho-hydroxybenzoic acid,heating the reaction product under sub-atmosphefic pnessiiu'e wherebyttis completely dehydrated, heating the dehydrated product to atemperature of about 240 C. in a substantially oxygen-free atmosphere,acidifying the product lime'ters of mercury to dehydrate completely thesaid. hydrated reaction product, heating the dehydrated product to atemperature of about 240 C. in a substantially men-free atmosphere,acidifidng the product of the preceding :atep whereby the saidpara-acid.mohtained, aratingthe para-acid from the modest liquor.

6. The process for the m'epmfion of para-hy- 1m droxy-benzoic acid whichcommeaciilfliwtassium hydroxide with 'oithoiepdrmbenznic acid, heatingthe hydrated reectionprodlwtto a temperature 01' between about 180end-39rd}. under an .absolutemressure between about 25 and 55finmillimeters oi mercmytodehydrateeompletaly the said hydrated reactionproduct, heating ithe dehydrated product to "a temperature oi about 240C. ina substantially oxygen-dme atmosphere, acidifying the product ofthe preceding step wherehythe said pom-midis obtoined'aad separating theparamcid 1km.

Thepeocess forthepmmratipnot pan-mydrooy-heamicacidwhicb @9- tossiumhydromde with art 6 w a a beteaoic acidiieating the hydrated reactionproduct to a temperature of between about 189? 260 10. under anabsolutepressure 25 ad 50 millimeters oi mercury to dehydrate completely thesaidhydrated 'reaotioniproduct, mating the dehydrated product to atempemturehetween about 240 and 280 0. ins. submntlaily oxygen-tree are,.addisigprthmhydrmwbenzoic acid to-the product of the preeed-ingstepwhereby about one-half of the potassium-therein 125 is replaced withhydrogen and potassium orthohydroxybenzoate .fomned, adding hydrochloricacid to the mixture of=thepreceding stepwhereby the remaining potassiumis replaced auditiotassium chloride formed, separatingparahydroxybenzoic acid from the reaction mixture, separatingthepotassium ortho-hydrosn benzoate from the potassium chloride motherliquor, and returning the potassium ortho-hydroxybenaoate to the firststep of theprooess.

135 LINDIEY E. m.

